In the FAQ and the Instructions it says that special abilities work regardless of facing except if the card says so.

So does that mean the purple space marine can attack 3 times regardless of facing ?

And the flame thrower can burn genestealers behind him?

Logically I would say no.

I'm sorry, I did not find this in either the faq or the instructions. Certain attack cards from teams such as brown and yellow ignore facing; it specifically states this. I assume any other time facing must be respected in regards to attacking.

If you give me a page number and a quote, I will check again.

Yes, special abilities ignore facing UNLESS the card specifically says otherwise (Source: FAQ pdf).

But note in the case of the Assault Cannon and Heavy Flamer abilities are when resolving an attack (Leon's is attack three times rather than just once, Zael's is when resolving an attack). Therefore they are still under the same attack restrictions that require the swarm to be in front of them to resolve an attack against it. 

 I'd agree with the post above. The wording on the card could be clarified, but it's during an attack, so something that is eligible for support tokens and must have a target and be targeted legally beforehand.